PURPOSE: To obtain easily a curved pipe having an equal wall thickness and high accuracy by forming a round bar-shaped blank material into a straight pipe- shaped semiprocessed product of which the pipe part at the intermediate between the end parts having a prescribed shape is provided with the uneven wall thickness to be made equal by curving and curving such product.

CONSTITUTION: A desired pipe hole 1 is opened in a round bar-shaped blank material M by forging, grinding, etc. over the entire length, thus forming bulging parts 2, 3 for connection at both ends. The uneven thickness to the extent that the part to be positioned on the inside in the stage of bending is thin and is increasingly thickly walled toward the outside and that these parts are made equal in thickness on account of the wall deformation occuring in the compressing force and stretching force in the axial line direction to be generated in the stage of bending the pipe part 4 between the parts 2, 3 is provided in said pipe part. The part 4 of the straight pipe-shaped semiprocessed product N formed in such a way is curved to an arc shape of about 90° and an intended finished product N' is obtd.